Microscopic Model for High-spin vs. Low-spin ground state in [Ni2M(CN)8][Ni_2{M(CN)_8]} (M=MoV,WV,NbIVM=Mo^V, W^V, Nb^{IV}) magnetic clusters

Conventional superexchange rules predict ferromagnetic exchange interaction between Ni(II) and M (M=Mo(V), W(V), Nb(IV)). Recent experiments show that in some systems this superexchange is antiferromagnetic. To understand this feature, in this paper we develop a microscopic model for Ni(II)-M systems and solve it exactly using a valence bond approach. We identify the direct exchange coupling, the splitting of the magnetic orbitals and the inter-orbital electron repulsions, on the M site as the parameters which control the ground state spin of various clusters of the Ni(II)-M system. We present quantum phase diagrams which delineate the high-spin and low-spin ground states in the parameter space. We fit the spin gap to a spin Hamiltonian and extract the effective exchange constant within the experimentally observed range, for reasonable parameter values. We also find a region in the parameter space where an intermediate spin state is the ground state. These results indicate that the spin spectrum of the microscopic model cannot be reproduced by a simple Heisenberg exchange Hamiltonian.Comment: 8 pages including 7 figure

Spin-crossover compounds based on iron(II) complexes of 2,6-bis(pyrazol-1-yl)pyridine (bpp) functionalized with carboxylic acid and ethyl carboxylic acid

Four new salts of the iron(II) complex of the 2,6-bis( pyrazol-1-yl)pyridine ligand functionalized with a carboxylic acid group (bppCOOH) of formulas [Fe(bppCOOH)2](BF4)2 (1(BF4)2), [Fe(bppCOOH)2] (CF3SO3)2·yMe2CO (1(CF3SO3)2·yMe2CO), [Fe(bppCOOH)2](AsF6)2·yMe2CO (1(AsF6)2·yMe2CO) and [Fe (bppCOOH)2](SbF6)2·yMe2CO (1(SbF6)2·yMe2CO) have been prepared and characterized together with a more complete structural and photomagnetic characterization of the previously reported [Fe (bppCOOH)2](ClO4)2 (1(ClO4)2). Furthermore, the iron(II) complex of the ethyl ester derivative of bppCOOH (bppCOOEt) has been prepared and characterized (compound [Fe(bppCOOEt)2] (ClO4)2·yMe2CO, 2(ClO4)2·yMe2CO). Isostructural 1(BF4)2 and 1(ClO4)2 show an abrupt and reversible spin transition with a much lower T1/2 for the BF4− salt. CF3SO3 , SbF6 and AsF6 counteranions and the bppCOOEt ligand lead to the incorporation of solvent molecules in the structures, which play an important role in the spin-crossover properties of these compounds. In the case of 1(CF3SO3)2·yMe2CO, a spin transition is obtained after desolvation. All these compounds show a LIESST effect

SYSTEMES A PRECURSEURS CYANOMETALLATES (COUCHES MINCES A HAUTES TEMPERATURES DE CURIE ET DERIVES D'OCTACYANOMETALLATES)

LA PREMIERE PARTIE DU TRAVAIL DE THESE A CONSISTE A METTRE SOUS FORME DE COUCHES MINCES MICRONIQUES UN ANALOGUE DU BLEU DE PRUSSE, CHROMICYANURE DE VANADIUM DONT LA TEMPERATURE DE CURIE PEUT ATTEINDRE 376 K. CETTE MISE EN FORME S'EST EFFECTUEE PAR ELECTROCHIMIE. LES COUCHES MINCES OBTENUES PRESENTENT DES TEMPERATURES DE CURIE T C QUI ATTEIGNENT 330 K. DES MESURES DE MAGNETO-OPTIQUE REALISEES DE 2 K A 373 K METTENT EN EVIDENCE LE CARACTERE POLYPHASE DU COMPOSE ET LA PRESENCE D'UNE HYSTERESIS MAGNETO-OPTIQUE A 373 K (0\C). NOUS MONTRONS L'INTERET ET LES LIMITES DE L'UTILISATION DES IONS DES DEUXIEME ET TROISIEME SERIES DES METAUX DE TRANSITION (CARACTERE DIFFUS DES ORBITALES D, EFFETS RELATIVISTES) POUR OBTENIR DES INTERACTIONS D'ECHANGE MAGNETIQUE OU DES ANISOTROPIES PLUS ELEVEES QUE POUR LES IONS DES METAUX DE LA PREMIERE SERIE DE TRANSITION. DANS LA SECONDE PARTIE DU MANUSCRIT, NOUS DECRIVONS LES STRUCTURES ET LES PROPRIETES DE COMPLEXES SYNTHETISES A PARTIR DE PRECURSEURS OCTACYANOMETALLATE M(CN) 8 Q , M = NB I V, MO I V, W I V, W V. NOUS DECRIVONS UNE STRUCTURE TRIDIMENSIONNELLE PRESENTANT UNE TEMPERATURE DE CURIE DE 50 K ENVIRON, UNE CHAINE DE TOPOLOGIE ORIGINALE PRESENTANT UNE ALTERNANCE D'INTERACTIONS FERRO- ET ANTIFERROMAGNETIQUES ET UNE MOLECULE A HAUT SPIN (S = 13/2). DES SYNTHESES DE MOLECULES A PARTIR DE PRECURSEURS OCTACYANOMETALLATE ET DE CATIONS PERIPHERIQUES M I IL 5(H 2O) 2 + (M = CO, NI ; L 5 LIGAND PENTADENTE) SONT DECRITES.PARIS-BIUSJ-Thèses (751052125) / SudocCentre Technique Livre Ens. Sup. (774682301) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Etude des propriétés optiques, magnétiques et photo-induites dans les matériaux à transition de spin (effets de la dilution de l'ion métallique)

De nombreux travaux ont été dédiés ces dernières années aux matériaux à transition de spin qui présentent des potentialités pour le stockage de l information en raison de leurs propriétés physiques commutables : métastabilité thermique et optique. Ce travail est relatif au mécanisme photo-induit dans les systèmes à transition de spin et, plus précisément, à l étude des effets de dilution sur la température LIESST. L objectif principal est de mieux cerner les paramètres qui gouvernent T(LIESST). Diverses études expérimentales ont été réalisées sur des composés mononucléaires 0D, polymériques 1D et polynucléaires. L analyse de l ensemble des propriétés photomagnétiques a permis de montrer que T(LIESST) est indépendant de la dilution et suggère que cette grandeur est gérée par la molécule et dépend très peu des interactions intermoléculaires.In the last few years, many studies have been dedicated to spin-crossover compounds because of their potential application in data-storage systems. In such systems, optical switching is particularly promising. This work deals with the photo-induced phenomenon encountered in spin crossover systems, more specifically, the effects of metal dilution on the LIESST temperature. The main objective is to better define the parameters that govern T (LIESST). Various experimental studies were performed on mononuclear 0D compounds, polymeric 1D systems and polynuclear systems. The analysis of all photomagnetic properties has shown that T(LIESST) is not affected by the dilution and suggests that this parameter is govern at the molecular scale and depends very few of intermolecular interactions.BORDEAUX1-Bib.electronique (335229901) / SudocSudocFranceF

Optimization of bistable molecular materials

Les complexes à transition de spin sont des molécules qui peuvent être commutées entre deux états, l un diamagnétique et l autre paramagnétique. Cette commutation peut s effectuer, entre autre, à l aide d une excitation lumineuse, ouvrant la voie vers un possible stockage de l information au niveau d une molécule unique. Toutefois, l information photo-inscrite n est stable qu au-dessous d une certaine température appelée T(LIESST). L objectif principal de cette thèse a constitué en des modifications chimiques (modification du ligand organique, désolvatation, modifications d anion, ) d un complexe macrocyclique de Fe(II) qui présente à l heure actuelle l un des T(LIESST) les plus élevés (134 K), et ce afin d augmenter encore cette valeur. Parmi les divers résultats expérimentaux, un T(LIESST) de 160 K a été obtenu.Spin transition complexes are molecules that can be commuted between two different states, one being diamagnetic and the other one paramagnetic. This commutation can be triggered by different ways, including light irradiation, opening a way to the storage of information at molecular level. However, the photo-inscribed information is stable only below a given temperature called T(LIESST). The main goal of this thesis has been to modify chemically (modification of organic ligand, desolvation, modification of anions, ) a Fe(II) macrocyclic ligand presenting one of the highest T(LIESST) known today (134 K) with the aim to further increase this temperature. Among the different results obtained, a T(LIESST) of 160 K has been reached.BORDEAUX1-Bib.electronique (335229901) / SudocSudocFranceF

Light-induced spin crossover—Solution and solid-state processes

This article overviews the different light-induced spin-crossover processes that are induced using ligand photoreaction, photothermal effects, or optical absorption of the center metallic ion. A particular attention is drawn on the light-induced excited spin-state trapping effect, the most studied phenomenon, by discussing the recent developments in terms of description of the initial photoswitching process and the mechanisms developing at different time scales, as well as the lifetime of the photoinduced state

Stepwise spin transition in a mononuclear iron(II) complex with unusually wide plateau

The reaction of the Schiff-base like ligand H2L [L2- = (E,E)-({diethyl 2,2-[1,2-phenylenebis(iminomethylidyne)]bis(3-oxobutanato)})] with iron(II) acetate in methanol in the presence of 4-cyanopyridine leads to the formation of an octahedral iron(II) complex with the formula FeL(CNpy)2·0.25CNpy (1). The magnetic properties of this complex were investigated using temperature-dependent susceptibility measurements (SQUID) and Mössbauer spectroscopy. Both measurements indicate that the complex performs a stepwise spin transition with a large plateau in the region from 225 K to 125 K and HS 0.25. The molecule structure of the compound was determined at room temperature, 240 K (HS 0.5 according to susceptibility measurements) and 100 K. Two complex molecules were found in the asymmetric unit with strong - interactions between the phenylene rings of H2L. In agreement with outcomes from the magnetic measurements, both iron centres are in the high-spin state at room temperature and in the low-spin state at 100 K. At 240 K one of the two iron centres is high-spin and the other one low-spin. The reason for the wide step with one fourth of the iron centres in the high-spin state is the additional distorted 4-cyanopyridine in the crystal that influences the transition temperature of the surrounding iron centres. The spin transition was followed in solution by using 1H NMR spectroscopy. A gradual spin transition with T1/2 = 238 K is obtained. Temperature-dependent diffuse reflectivity spectra show the occurrence of photoconversion at the surface of the sample but only about 6 % of the complex could be switched in bulk condition using a SQUID magnetometer and the stability of the photoinduced HS state with TLIESST = 40 K is very weak

Octadecanuclear cluster or 1D polymer with [{ML}2Nb(CN)8]n motifs as a function of {ML} (M = Ni(II), n = 6; M = Mn(II), n = ¥; L = macrocycle)

A nanosized octadecaheteronuclear aggregate, [{NiL2}12{Nb-(CN)8}6(H2O)6], and a 1-D coordination polymer, [{MnL1}2{Nb-(CN)8}(H2O)]¥, have been obtained by self-assembly between the octacyanometalate {Nb(CN)8}4- and {ML}2+ complexes..